An expression is obtained for the variance of overflow traffic from a fully-accessible channel group, when offered Bernoulli-Poisson-Pascal (BPP) traffic. The formula is exact within the context of Delbrouck's BPP method and provides an alternative to approximations and numerical-based methods for evaluation of the variance. Applications include two-moment blocking network analysis methods, where switch overflow traffic is not lost but offered on alternative routes, for example in hierarchical cellular networks and optically switched networks with alternative routing schemes.